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Chapter 2
PART DESIGN SPECIFICATION
Dr. T.C. Chang
School of Industrial EngineeringPurdue University
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THE DESIGN PROCESS
Design Process
1. Conceptualization
2. Synthesis3. Analysis
4. Evaluation
5. Representation
Design Process
(VDI)
1. Clarification of the task
2. Conceptual design
3. Embodiment design
4. Detailed design
Functional requirement -> Design
Steps 1 & 2 needs creativity, sketch is sufficient3 mathematical, engineering analysis4 simulation, cost, physical model5 formal drawing or modeling
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DESIGN REPRESENTATION
Design Representation Manufac-turing
Verbal
Sketch Multiview orthographic drawing (drafting)
CAD drafting
CAD 3D & surface model
Solid model
Feature based design
Requirement of the representation method
precisely convey the design concept
easy to use
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A FREE-HAND SKETCH
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A FORMAL DRAWING
0.9444"
4 holes 1 /4" dia
arou nd 2 " dia , first
hole at 45
A
2.0000.001
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DESIGN DRAFTING
Third angle projection
Profile plane
Y
Z
X
I I I
Horizontal
Frontal plane
I
I V
I I
top
front
side
a
b
c d ef
g
h i
j
Drafting in the third angle
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INTERPRETING A DRAWING
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DESIGN DRAFTING
Partial view
Cut off view and auxiliary view
Provide more local details
A
2.0000.001
A
A
A - A
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DIMENSIONING
Requirements1. Unambiguous
2. Completeness
3. No redundancy0.83 ' 0.95 ' 1.22 '
3.03 '
Redundant dimensioning
0.83 ' 1.22 '
3.03 '
1.72 '
0.86 '
Adequate dimensioning
Incompletedimensioning
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TOLERANCE
Dimensional tolerance - conventional
Geometric tolerance - modern
unilateral
bilateral
1.00 0.05+-
nominal dimension
tolerance
0.95+ 0.10- 0.00 1.05
+ 0.00- 0.10
1.00 0.05+-
0.95 - 1.05means a range
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TOLERANCE STACKING
"TOLERANCE IS ALWAYS ADDITIVE" why?
What is the expected dimension and tolerances?
d = 0.80 +1.00 + 1.20 = 3.00
t = (0.01 + 0.01 + 0.01) = 0.03
0.80 ' 0.01 1.20 ' 0.01
1.00 ' 0.01
?
1. Check that the tolerance & dimension specifications arereasonable - for assembly.
2. Check there is no over or under specification.
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TOLERANCE STACKING (ii)
What is the expected dimension and tolerances?
d = 3.00 - 0.80 - 1.20 = 1.00
t = (0.01 + 0.01 + 0.01) = 0.03
0.80 ' 0.01 1.20 ' 0.01
3.00 ' 0.01
?
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TOLERANCE STACKING (iii)
Maximum x length = 3.01 - 0.79 - 1.19 = 1.03Minimum x length = 2.99 - 0.81 - 1.21 = 0.97
Therefore x = 1.00 0.03
0.80 ' 0.01 1.20 ' 0.01
3.00 ' 0.01
?
x
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TOLERANCE GRAPH
G(N,d,t)
N: a set of reference lines, sequenced nodes
d: a set of dimensions, arcs
t: a set of tolerances, arcs
A B C D Ed,td,t d,t
d,t
d : dimension between references i & j
t : tolerance between references i & jij
ij
Reference i is in front of reference j in the sequence.
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EXAMPLE TOLERANCE GRAPH
A B C D E
A B C D Ed,t d,t d,t
d,t
different propertiesbetween d & t
dDE= d
DA+ d
AE=
dAD+ d
AE
= (dAB+ d
BC+ d
CD) + d
AE
tDE= tAB+ tBC+ tCD + tAE
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OVER SPECIFICATIONIf one or more cycles can be detected in the graph, we say that the
dimension and tolerance are over specified.
A B C
A B C
A B C
d1 d2
d3d1,t1 d2,t2
d3,t3
t1 t2
t3
Redundant dimension
Over constraining tolerance(impossible to satisfy) why?
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UNDER SPECIFICATION
A B C D E
A B C D Ed1 d2
d3
C D is disconnected from the
rest of the graph.
No way to find dBC and dDE
When one or more nodes are disconnected from the graph, the
dimension or tolerance is under specified.
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PROPERLY TOLERANCED
A B C D E
A B C D Ed,t d,t d,t
d,t
dDE= d
DA+ d
AE= d
AD+ d
AE
= (dAB+ d
BC+ d
CD) + d
AE
tDE= tAB+ tBC+ tCD + tAE
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TOLERANCE ANALYSIS
For two or three dimensional tolerance analysis:
i. Only dimensional tolerance
Do one dimension at a time.
Decompose into X,Y,Z, three one dimensional problems.
ii. with geometric tolerance
? Don't have a good solution yet. Use simulation?
t rue posit ion
diamet er & t olerance
A circular tolerance zone, the size is influencedby the diameter of the hole. The shape of thehole is also defined by a geometric tolerance.
C C
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3-D GEOMETRIC TOLERANCEPROBLEMS
t
datum surfacedatumsurface
Referenceframe
perpendicularity
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TOLERANCE ASSIGNMENT
Tolerance is money
Specify as large a tolerance as possible as long as functional andassembly requirements can be satisfied.
(ref. Tuguchi, ElSayed, Hsiang, Quality Engineering in ProductionSystems, McGraw Hill, 1989.)
function
cost
Tolerance value
d (nominal dimension)
Quality
Cost
-t
+t
Quality cost
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REASON OF HAVING TOLERANCE
No manufacturing process is perfect.
Nominal dimension (the "d" value) can not beachieved exactly.
Without tolerance we lose the control and as aconsequence cause functional or assemblyfailure.
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EFFECTS OF TOLERANCE (I)
1. Functional constraintse.g.
d t
flow rate
Diameter of the tube affects the flow. What is the allowedflow rate variation (tolerance)?
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EFFECTS OF TOLERANCE (II)
2. Assembly constraintse.g. peg-in-a-hole dp
dh
How to maintain theclearance?
Compound fitting
The dimension ofeach segmentaffects others.
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RELATION BETWEENPRODUCT & PROCESS
TOLERANCES
Setup
locators
0.005
0.005
0.005
Design specifications
Process tolerance
Machine uses the locators asthe reference. The distancesfrom the machine coordinatesystem to the locators are
known. The machining tolerance is
measured from the locators.
In order to achieve the 0.01tolerances, the processtolerance must be 0.005 or
better. When multiple setups are used,
the setup error need to be takeninto consideration.
A
0 . 0 1 t o l e r an c e s
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TOLERANCE CHARTINGA method to allocate process tolerance and verify that the process
sequence and machine selection can satisfy the design tolerance.
0 .01 0 .0 1
0 .01
stock
boundary
Dim t ol
1.0 0.01
1.0 0.01
3.0 0.01
Op code
10 lathe
10 lathe
20 lathe
20 lathe
10
12
20
22
blue print
Operationsequence
Not shown areprocess toleranceassignment andbalance
produced tolerances:
process tol of 10 + process tol of 12
process tol of 20 + process tol 22
process tol of 22 + setup tol
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SURFACE FINISH
waviness width
roughness width
waviness
roughness
63 0.010
0.005
0.002 - 2roughnessheight
waviness height
waviness width
roughness width cutoffdefault is 0.03" (ANSI Y14.36-1978)roughness width
Lay
( inch)
(inch)
63
Usuallysimplified:
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PROBLEMS WITH DIMENSIONALTOLERANCE ALONE
1.001
1.0011.001
6.00
1.000.001
6.000.001
As designed:
As manufactured:
Will you accept the partat right?
Problem is the control ofstraightness.
How to eliminate theambiguity?
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GEOMETRIC TOLERANCES
FORM
straightness
flatness
Circularity
cylindricity
ORIENTATION
perpendicularity
angularity
parallelism
LOCATION
concentricity
true positionsymmetry
RUNOUT
circular runouttotal runout
PROFILEprofileprofile of a line
ANSI Y14.5M-1994 GD&T (ISO 1101, geometric tolerancing;
ISO 5458 positional tolerancing; ISO 5459 datums;and others)
Squareness
roundness
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g
DATUM &FEATURE CONTROL FRAME
Datum: a reference plane, point, line, axis where usually a planewhere you can base your measurement.
Symbol:
Even a hole pattern can be used as datum.
Feature: specific component portions of a part and may include oneor more surfaces such as holes, faces, screw threads, profiles, orslots.
Feature Control Frame:
A
// 0.005 M A
symbol tolerance value
modifier
datum
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MODIFIERS
M Maximum material condition MMC assembly
Regardless of feature size RFS (implied unless specified)
L Least material condition LMC less frequently used
P Projected tolerance zone
O Diametrical tolerance zone
T Tangent plane
F Free state
maintain critical
wall thickness orcritical location offeatures.
MMC, RFS, LMC
MMC, RFS
RFS
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SOME TERMS
MMC : Maximum Material ConditionSmallest hole or largest peg (more material left on the part)
LMC : Least Material Condition
Largest hole or smallest peg (less material left on the part)
Virtual condition:
Collective effect of all tolerances specified on a feature.
Datum target points:
Specify on the drawing exactly where the datum contact pointsshould be located. Three for primary datum, two for secondarydatum and one or tertiary datum.
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DATUM REFERENCE FRAMEThree perfect planes used to
locate the imperfect part.a. Three point contact on the
primary plane
b. two point contact on thesecondary plane
c. one point contact on the tertiaryplane
O 0.001 M A B C
primary Secondary
Tertiary
Primary
Secondary
Te r t iar y
A
B
C
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STRAIGHTNESS
Value must be smallerthan the size tolerance.
1.000 ' 0.002
0.001
Measured error 0.001
1.000 ' 0.002
0.001
0.001
Design Meaning
Tolerance zone between two straightness lines.
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FLATNESS
1.000 ' 0.002
0.001
0.001
parallel
planes
Tolerance zone defined by two parallel planes.
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CIRCULARITY (ROUNDNESS)
1.00 ' 0.05
0.01
At any section along the cylinder
a. Circle as a result of the intersection by any plane perpendicular to
a common axis.b. On a sphere, any plane passes through a common center.
Tolerance zone bounded by two concentric circles.
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CYLINDRICITY
1.00 ' 0.05
0.01
0.01
Rotate in a V
Rotate between points
Tolerance zone bounded by two concentric cylinderswithin which the cylinder must lie.
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PERPENDICULARITYA surface, median plane, or axis at a right angle to the datum plane
or axis.
1.000 ' 0.005
. 0 0 2 A
0.500 ' 0.005
2.000 ' 0.005
A
. 0 0 2 T A
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ANGULARITYA surface or axis at a specified angle (orther than 90) from a datum
plane or axis. Can have more than one datum.
3.500 ' 0.005
1.5000.005
40
0.005 A
A
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PARALLELISM
1.000 " 0.005
2.000 " 0.005
.001 A
A
The condition of a surface equidistant at all points from a datum plane,or an axis equidistant along its length to a datum axis.
0.001
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PROFILEA uniform boundary along the true profile within whcih
the elements of the surface must lie.
A
B
0.005 A B
0.001
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RUNOUT
0.361 " 0.002
1.500 " 0.005A
0.005 A
A composite tolerance used to control the functional relationshipof one or more features of a part to a datum axis. Circular runout
controls the circular elements of a surface. As the part rotates360 about the datum axis, the error must be within the tolerancelimit.
Datum
axis
Deviation on each
circular check ring
is less than the
tolerance.
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TOTAL RUNOUT
Datum
axis
Deviation on the
total swept when
the part is rotating
is less than the
tolerance.
0.361 " 0.002
1.500 " 0.005A
0.005 A
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TRUE POSITION
1.20
0.01
1.00 0.01
1.20
1.00
Tolerance zone
0.01dia
O0.01MA B
O.800.02
Dimensionaltolerance
True positiontolerance
Hole center tolerance zone
A
B
Tolerance zone0.022
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HOLE TOLERANCE ZONE
Tolerance zone for dimensional tolerancedhole is not a circle. This causes some assemblyproblems.
For a hole using true position tolerancethe tolerance zone is a circular zone.
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TOLERANCE VALUE MODIFICATION
Produced True Pos tol
hole size0.97 out of diametric tolerance
0.98 0.01 0.05 0.01
0.99 0.02 0.04 0.01
1.00 0.03 0.03 0.01
1.01 0.04 0.02 0.01
1.02 0.05 0.01 0.01
1.03 out of diametric tolerance
1.20
1.00
O 0.01 M A B
O 1.00 0.02
M L S
The default modifier fortrue position is MMC.
MMC
LMC
For M the allowable tolerance = specified tolerance + (produced holesize - MMC hole size)
A
B
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MMC HOLE
Given the same peg (MMC peg), when the produced hole sizeis greater than the MMC hole, the hole axis true positiontolerance zone can be enlarged by the amount of differencebetween the produced hole size and the MMC hole size.
hole axis tolerance zone
MMC holeLMC hole
MMC peg will fit in t he hole
axis must be in the tolerance zone,
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PROJECTED TOLERANCE ZONEApplied for threaded holes or press fit holes to ensure interchangeability
between parts. The height of the projected tolerance zone is the thicknessof the mating part.
O .01 0 M A B C
.250 p
.375 - 16 UNC - 2B
Project ed t olerance
zone0.25
0.01
Produced part
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SOME NUMBERSKrulikowski, A., GD&T Challenges the Fast Draw, MFG ENG, feb 1994.
GD&T drawings are more expansive to make, however, saves revisioncost.
Drawing revision costs $500 - $2000 on the paper work